The words G. nig and nigla “a mouse” appeared in the Gnomish Lexicon of the 1910s above the verbs G. nigla- “thieve” and G. nig- “steal” (GL/60), probably derived from a variant of the early root ᴱ√NAQA “steal” (QL/64).
Neo-Sindarin: I would adapt this word into Neo-Sindarin as ᴺS. nigol “mouse”, reconceived of as elaboration of √NIK “small”.
A verb for “steal, creep, do or go by stealth” in the Gnomish Lexicon of the 1910s with past form naigi (GL/60), probably based on the early root ᴱ√NAQA “steal, take; get by stealth, unlawfully” or a variant thereof (QL/64).